netdev - Re: [PATCH bpf] flow_dissector: Drop BPF flow dissector prog ref on netns cleanup

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Message-ID: <CAADnVQKztsp-fF5Mbi7DUGrM-5SfH24xntaF0Qaewxr9ax7ZRw@mail.gmail.com>
Date:   Thu, 21 May 2020 17:53:14 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jakub Sitnicki <jakub@...udflare.com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...udflare.com>,
        Petar Penkov <ppenkov@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>
Subject: Re: [PATCH bpf] flow_dissector: Drop BPF flow dissector prog ref on
 netns cleanup

On Thu, May 21, 2020 at 12:09 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, May 20, 2020 at 10:24 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
> >
> > When attaching a flow dissector program to a network namespace with
> > bpf(BPF_PROG_ATTACH, ...) we grab a reference to bpf_prog.
> >
> > If netns gets destroyed while a flow dissector is still attached, and there
> > are no other references to the prog, we leak the reference and the program
> > remains loaded.
> >
> > Leak can be reproduced by running flow dissector tests from selftests/bpf:
> >
> >   # bpftool prog list
> >   # ./test_flow_dissector.sh
> >   ...
> >   selftests: test_flow_dissector [PASS]
> >   # bpftool prog list
> >   4: flow_dissector  name _dissect  tag e314084d332a5338  gpl
> >           loaded_at 2020-05-20T18:50:53+0200  uid 0
> >           xlated 552B  jited 355B  memlock 4096B  map_ids 3,4
> >           btf_id 4
> >   #
> >
> > Fix it by detaching the flow dissector program when netns is going away.
> >
> > Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> > Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> > ---
> >
> > Discovered while working on bpf_link support for netns-attached progs.
> > Looks like bpf tree material so pushing it out separately.
> >
> > -jkbs
> >
>
> [...]
>
> >  /**
> >   * __skb_flow_get_ports - extract the upper layer ports and return them
> >   * @skb: sk_buff to extract the ports from
> > @@ -1827,6 +1848,8 @@ EXPORT_SYMBOL(flow_keys_basic_dissector);
> >
> >  static int __init init_default_flow_dissectors(void)
> >  {
> > +       int err;
> > +
> >         skb_flow_dissector_init(&flow_keys_dissector,
> >                                 flow_keys_dissector_keys,
> >                                 ARRAY_SIZE(flow_keys_dissector_keys));
> > @@ -1836,7 +1859,11 @@ static int __init init_default_flow_dissectors(void)
> >         skb_flow_dissector_init(&flow_keys_basic_dissector,
> >                                 flow_keys_basic_dissector_keys,
> >                                 ARRAY_SIZE(flow_keys_basic_dissector_keys));
> > -       return 0;
> > +
> > +       err = register_pernet_subsys(&flow_dissector_pernet_ops);
> > +
> > +       WARN_ON(err);
>
> syzbot simulates memory allocation failures, which can bubble up here,
> so this WARN_ON will probably trigger. I wonder if this could be
> rewritten so that init fails, when registration fails? What are the
> consequences?

good catch. that warn is pointless.
I removed it and force pushed the bpf tree.